Most Probable:
1. The official names of our titular Legendaries.
They might just be Solgaleo and Lunaala, but they are subject to change, knowing the curveballs thrown at us in the past three months.
2.More verification on the new region's Professor.
Just to be absolutely certain what he looks like. The fact that another person shows us the new region's Starters has sparked confusion, so some verification on who those two officially are would be nice.
3.Two or more new Pokémon.
If I'm not mistaken, in June 2013, when X & Y were getting tonnes of new information revealed to public, the Pokémon Vivillon, Noivern, Skrelp, and Clauncher were revealed to us throughout the month.
Least Probable:
1. A selfish rival.
We're well overdue with one by now, since the nice rival thing seems to be done to the ground. A true jerk will motivate tonnes of players to be the very best, just like what Gary/Blue and Silver helped us to be back in the golden days. Personally, I'd like a rich upper class girl stereotype who has one of her parents blackmailing the professor, perhaps the lovechild of Pacifica Northwest and Silver? Those two come to mind, when it comes down to gradual character development over the course of the story and being biologically related to an influential antagonist behind closed doors.
2. Mega Evolutions or New Evolutions of Past Pokémon.
They might be revealed three to two months before the games' official release, since it may be a bit too early to reveal them at this point. I think Game Freak really wants to surprise us, given the scarce updates between February and May.
Most probable:
1) Definitely, I'd expect confirmation that the legends' names are Solgaleo and Lunaala, or their real names if they aren't. For types however, Xerneas and Yveltal's types were revealed by Corocoro in July 2013, so that isn't guaranteed.
2) Quite possible, but Sycamore was only revealed by Corocoro in July 2013 with the legends' types, so we may have to wait another month or two for them.
3) 2-3 new pokemon sounds quite possible. You are correct in that the pokemon you mentioned above were revealed in June 2013, but that wasn't all that was. Noivern, Vivillon and the Fairy-type were revealed at the E3 direct, then Clauncher, Skrelp and Talonflame were revealed in a trailer on the show floor alongside Horde and Sky Battles, then finally Corocoro revealed Flabebe, Scatterbug, Spewpa and Litleo. 2-3 pokemon is quite reasonable, even if we do get the full reveal of the legends.
Least probable:
1) Personally I think we'll get a group of rivals, one of them being a spoilt brat is quite possible. Calem/Serena could have been that, being the son/daughter of famous pokemon trainers, but you're right, we haven't had a good prick since Silver. It's a month or two too early for the rivals to be revealed though.
2) As I said above, we did get Horde battles, Sky Battles and the Fiary type revealed in June 2013. We could get a new mechanic like Synchro Evolution (Ash-Greninja) revealed this month. As for cross-gen evos, we did get Munchlax and Sylveon revealed near the start of their gen's respective prerelease. We could get a cross-gen evo revealed here (maybe 2 if we're really lucky, but personally I think the slot would be better reserved for a brand new pokemon). New megas, in a month or two, I'd say.